FIG. 1 is a block diagram of some of the elements of telephone network 100. Telephone 110 connects to service switching point (SSP) 120 through subscriber line 130. Typically, subscriber line 130 is a twisted-pair of copper wires. SSP 120 contacts remote SSP 140 through signal transfer point (STP) 135 to reserve an idle trunk circuit. Direct communication between SSP 120 and remote SSP 140 takes place over trunk circuit 150. A node is any element of a telecommunications network that establishes voice or data links for the users of the network (i.e., a SSP, STP, etc.).
A network operator assigns each node within network 100 a point code. Elements within network 100 use point codes to identify the source and destination points of a message. Each node within network 100 identifies itself with an originating point code (OPC) and identifies other nodes within the network with a destination point code (DPC). The network operator also assigns a network identifier to network 100 to facilitate communication between network 100 and other networks.
There are a finite number of communication circuits between nodes in system 100. Circuit identification codes (CICs) are used to partially identify the communication channels between nodes. Networks employing the International Telecommunications Union Telecommunication Standardization Section (ITU-T)—Signaling System No. 7 (Q.761-Q.764 standard (ITU Standard) use a twelve bit CIC. Similarly, networks employing the American National Standards Institute—Signaling System No. 7 (T1.113) standard (ANSI Standard) use a fourteen bit CIC. Nodes using a twelve bit CIC are able to identify 4096 (2^12) communication channels and nodes using a fourteen bit CIC are able to identify 16,384 (2^14) communication channels.
The SS7 standard defines protocols that are used to implement basic call setup, management, and tear down. The protocols defined by the SS7 standard are divided into abstractions called levels. The SS7 levels loosely map to the Open Systems Interconnect (OSI) 7-layer model defined by the International Standards Organization (ISO).
FIG. 2 shows a comparison between the OSI 7-layer model and the SS7 levels. Integrated Services Digital Network User Part (ISUP) layer 210 manages connections between two SSPs. ISUP 210 implements call management by exchanging message signal units (MSUs) with the ISUP layer in a remote node. ISUP information is carried in the signaling information field (SIF) of the MSU. The SIF contains the OPC of the node sending the message and the DPC of the node that is the intended recipient of the MSU.
Message Transfer Part 3 (MTP3) layer 220 provides service to ISUP 210. MTP3 220 uses the routing information in a SIF to provide ISUP 210 with the OPC and DPC that identifies which node sent the MSU and which node is the intended recipient of the MSU. MTP3 220 also provides ISUP 210 with the CIC code that partially identifies the communication channel associated with the MSU.